It is a long-standing practice in the design and manufacture of earth-boring bits to apply wear-resistant hardfacing materials to the surfaces of such bits that are subjected to abrasive wear during drilling operations. In earth-boring bits of the rolling cutter variety, these surfaces include the teeth of bits of the milled or steel tooth variety, the gage surfaces of the rolling cutters and the shirttails of the bit legs comprising the bit body.
In the past, these hardfacing compositions generally comprise carbides of the elements of Groups IVB, VB and VIB in a matrix metal of iron, cobalt or nickel and alloys and mixtures thereof. The hardfacing is applied by melting the matrix and a portion of the surface to which the hardfacing is applied with an oxyacetylene or atomic hydrogen torch. The carbide particles give the hardfacing material hardness and wear resistance, while the matrix metal lends the hardfacing fracture toughness. A hardfacing composition must strike an adequate balance between wear resistance (hardness) and fracture toughness. A hardfacing composition that is extremely hard and wear-resistant may lack fracture toughness, causing the hardfacing to crack and flake prematurely. Conversely, a hardfacing with adequate fracture toughness, but inadequate hardness and wear resistance, is eroded prematurely and fails to serve its purpose.
Many factors affect the suitability of a hardfacing composition for a particular application. These factors include the chemical composition and physical structure of the carbides employed in the composition, the chemical composition and microstructure of the matrix metal or alloy, and the relative proportions of the carbide materials to one another and to the matrix metal or alloy.
One early advance in hardfacing compositions for use in earth-boring bits is disclosed in commonly assigned U.S. Pat. No. 3,800,891, Apr. 2, 1974 to White et al. This patent discloses a hardfacing composition comprising sintered tungsten carbide in an alloy steel matrix. Sintered tungsten carbide comprises grains or particles of tungsten carbide sintered with and held together by a binder of non-carbide material, such as cobalt. The sintered tungsten carbide possesses greater fracture toughness than the more conventional cast tungsten carbide, such that the resulting hardfacing composition possess good fracture toughness without sacrificing hardness and wear resistance.
U.S. Pat. No. 4,836,307, Jun. 6, 1989 to Keshavan et al., discloses a hardfacing composition employing particles of cemented or sintered tungsten carbide and relatively small particles of single crystal monotungsten carbide, sometimes referred to as "macrocrystalline" tungsten carbide, in a mild steel matrix. This composition purports to possess the advantages of sintered tungsten carbide, as disclosed in U.S. Pat. No. 3,800,891, with the advantages of single crystal monotungsten carbide, which is harder than the cemented or sintered tungsten carbide, yet is less brittle than the alternative cast carbide.
U.S. Pat. No. 5,089,182, Feb. 18, 1992, to Findeisen, et al. discloses a method of manufacturing cast carbide pellets that are generally spherical in shape and have improved mechanical and metallurgical properties over prior-art carbide pellets. These cast pellets are not truly spherical, but are sufficiently symmetrical that residual stresses in the pellets are minimized. Also, the generally spherical shape of these pellets eliminates corners, sharp edges and angular projections, which are present in conventional crushed particles, that increase residual stresses in the particles and tend to melt as the hardfacing composition is applied to the surface.
U.S. Pat. No. 5,663,512, Sep. 2, 1997 to Schader et al. discloses a hardfacing composition which includes a quantity of spherical sintered tungsten carbide granules and a quantity of cast spherical cast tungsten carbide granules in a eutectic form of WC/W.sub.2 C.
During application, some melting of the sintered spherical carbide granules occurs, which precipitates into the metal matrix and coats the spherical WC/W.sub.2 C granules. This composition provides a good balance between hardness and fractures toughness.